A lithium secondary battery is generally manufactured using a positive electrode and a negative electrode including electrode active materials capable of intercalating/deintercalating lithium ions, and an electrolyte that is a transferring medium of the lithium ions.
As the electrolyte, a liquid state electrolyte, particularly, an ion conductive organic liquid electrolyte dissolving a salt in a non-aqueous organic solvent has been normally used in the art. However, such a liquid electrolyte has a possibility of leakage during operation, and has a disadvantage of causing ignition, explosion and the like due to high flammability of the non-aqueous organic solvent used.
Accordingly, in order to overcome a stability problem of a liquid electrolyte, a lithium secondary battery using a gel polymer electrolyte preventing leakage of an electrolyte liquid by containing an electrolyte liquid and a salt in a polymer or a solid polymer electrolyte formed only with a polymer and a salt has recently received attention.
As the polymer used in the solid polymer electrolyte, polyvinylidene fluoride (PVDF) series, polyacrylonitrile (PAN) series, polyethylene oxide (PEO) series, polymethyl methacrylate (PMMA) series, mixtures thereof or copolymers thereof may be included.
Meanwhile, when a low molecular weight polyethylene oxide polymer is used in preparing the solid polymer electrolyte, high ion conductivity (o) may be obtained at room temperature, however, there is a disadvantage of liquidization by the polymer being present with a salt. As a result, the use of high molecular weight polyethylene oxide has been required when preparing the solid polymer electrolyte.
However, using high molecular weight polyethylene oxide has a disadvantage in that ion conductivity decreases to 10−5 S/cm at room temperature while ion conductivity is relatively high of 10−4 S/cm at a high temperature of 60° C. or higher. In other words, lithium ion migration in the solid polymer electrolyte occurs by a segmentation movement of a polymer, and in the high molecular weight polyethylene oxide, such a movement is restricted due to high crystallinity causing a decrease in the ion conductivity.
In view of the above, development of a solid polymer electrolyte capable of obtaining both high ion conductivity and mechanical strength in a wide temperature range by suppressing crystallinity when using polyethylene oxide has been required.